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Old 06-05-2020, 10:32 AM   #8 (permalink)
aerohead
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Porsche 911 controversy

Quote:
Originally Posted by JulianEdgar View Post
Well, your reference starts off with a picture that I commissioned from an artist (Dave Heinrich), drawn on the basis of wool tuft testing that I carried out on the road in 2001 - see https://autospeed.com.au/cms/a_1065/article. So thanks for the compliment. (However, it is of a different 911 model, so it doesn't match the rest of the pics in your cited thread.)

So let's look at Anyone have 911 CFD Analysis - Pelican Parts Forums, the thread you cited.

The pics show, without the spoiler, a separation bubble on the upper part of the slightly recessed rear glass, with flow attachment then re-occurring.

With the spoiler fitted, the separation is much more widespread on the rear glass, and flow attachment does not re-occur.

We don't need to read all your theories to see exactly what is happening: the spoiler creates flow separation that reduces lift, exactly as would occur over any lifting shape.

That's the reality, that is obvious to any observer looking at the story of the tufts - and I encourage people to go look for themselves at these pics.

(And, as you often do, you're misquoting Hucho. Neither page reference supports what you claim of it.)
1) The tufts do not represent the actual flow conditions, they are an artifact of the induced downwash, created by the longitudinal vortices, which the tufts fail to represent.
2) The TBL on the 911 is already compromised, half way between the top of the backlight and roof apex. The 911's rear contour happens too early, and its downslope is too steep to support attached flow.
3) Since the backlight is at 26-degrees, there's no chance of reattachment. What your viewing is a tuft alignment due to the downwash already in place.
4) The duckbill spoiler is attempting to 'reach' up,out of the separated flow where the inviscid flow can attack it. It falls short, however the vortex-induced downwash is attacking it, aiding engine cooling, lift, and a tiny bit of drag.
5) Your concept of a 'lifting shape' is not supported by science. You know from your own experiments that, any major lift on the 911 would be ahead of the windshield header.
6) If you'll continue your studies, you'll learn that the 'lift' that you're interested in is caused by the region of separation-induced low pressure acting over the top majority of the Porsche's aft-body.
7) You won't understand this concept until you investigate Bernoulli's Theorem. Ask any of your four favorite aerodynamicists.
8) Any 'observer' who's passed a course in Fluid Mechanics would object to your explanation of the spoiler's function.
9) As to Hucho, all I can do is, list his quotes which are germane to this issue. I cannot comprehend them for anyone else.
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